Golf equipment, i.e., clubs and balls, are formed from a variety of compositions. For example, golf ball covers are formed from a variety of materials, including balata and ionomer resins. Balata is a natural or synthetic trans-polyisoprene rubber. Balata covered balls are favored by more highly skilled golfers because the softness of the cover allows the player to achieve spin rates sufficient to more precisely control ball direction and distance, particularly on shorter shots.
However, balata covered balls are easily damaged, and thus lack the durability required by the average golfer. Accordingly, alternative cover compositions have been developed in an attempt to provide balls with spin rates and a feel approaching those of balata covered balls, while also providing a golf ball with a higher durability and overall distance.
Ionomer resins have, to a large extent, replaced balata as a cover material. Chemically, ionomer resins are a copolymer of an olefin and an α,β-ethylenically-unsaturated carboxylic acid having 10 to 90 percent of the carboxylic acid groups neutralized by a metal ion, as disclosed in U.S. Pat. No. 3,264,272. Commercially available ionomer resins include, for example, copolymers of ethylene and methacrylic or acrylic acid, neutralized with metal salts. Examples of commercially available ionomer resins include, but are not limited to, SURLYN® from DuPont de Nemours and Company, and ESCOR® and IOTEK® from Exxon Corporation. These ionomer resins are distinguished by the type of metal ion, the amount of acid, and the degree of neutralization.
U.S. Pat. Nos. 3,454,280, 3,819,768, 4,323,247, 4,526,375, 4,884,814, and 4,911,451 all relate to the use of SURLYN®-type compositions in golf ball covers. However, while SURLYN® covered golf balls, as described in the preceding patents, possess virtually cut-proof covers, the spin and feel are inferior compared to balata covered balls.
Polyurethanes have also been recognized as useful materials for golf ball covers since about 1960. U.S. Pat. No. 3,147,324 is directed to a method of making a golf ball having a polyurethane cover. The resulting golf balls are durable, while at the same time maintaining the “feel” of a balata ball.
Various companies have investigated the usefulness of polyurethane as a golf ball cover material. U.S. Pat. No. 4,123,061 teaches a golf ball made from a polyurethane prepolymer formed of polyether with diisocyanate that is cured with either a polyol or an amine-type curing agent. U.S. Pat. No. 5,334,673 discloses the use of two categories of polyurethane available on the market, i.e., thermoset and thermoplastic polyurethanes, for forming golf ball covers and, in particular, thermoset polyurethane covered golf balls made from a composition of polyurethane prepolymer and a slow-reacting amine curing agent, and/or a glycol.
Unlike SURLYN® covered golf balls, polyurethane golf ball covers can be formulated to possess the soft “feel” of balata covered golf balls. However, golf ball covers made from polyurethane have not, to date, fully matched SURLYN® golf balls with respect to resilience or the rebound of the golf ball cover, which is a function of the initial velocity of a golf ball after impact with a golf club.
Furthermore, because the polyurethanes used to make the covers of such golf balls generally contain an aromatic component, e.g., aromatic diisocyanate, polyol, or polyamine, they are susceptible to discoloration upon exposure to light, particularly ultraviolet (UV) light. To slow down the discoloration, light and UV stabilizers, e.g., TINUVIN® 770, 765, and 328, are added to these aromatic polymeric materials. However, to further ensure that the covers formed from aromatic polyurethanes do not appear discolored, the covers are painted with white paint and then covered with a clear coat to maintain the white color of the golf ball. The application of a uniform white pigmented coat to the dimpled surface of the golf ball is a difficult process that adds time and costs to the manufacture of a golf ball.
In addition, while polyurethanes formed from polyether polyols are slightly more stable than polyurethanes formed using polyester polyols in terms of moisture resistance, polyurethanes are highly susceptible to changes in their physical properties due to absorption of moisture. To avoid moisture absorption, manufacturers have attempted to use moisture barrier layers, e.g., U.S. Pat. No. 5,820,488, located between the core and the cover. However, there still remains a need for materials that are resistant to absorption of moisture suitable for forming a golf ball component.
Polyureas have also been proposed as cover materials for golf balls. For instance, U.S. Pat. No. 5,484,870 discloses a polyurea composition comprising the reaction product of an organic isocyanate and an organic amine, each having at least two functional groups. Once these two ingredients are combined, the polyurea is formed, and thus the ability to vary the physical properties of the composition is limited. Like polyurethanes, polyureas are not completely comparable to SURLYN® golf balls with respect to resilience or the rebound or damping behavior of the golf ball cover.
Therefore, there remains a continuing need for golf equipment having soft components that provide improved resilience, increased cut, scratch and abrasion resistance, moisture resistance, and enhanced adherence without adversely affecting overall performance characteristics of the golf balls. In addition, it would be advantageous to provide a composition that combines the cut and scratch resistance with improved resistance to discoloration that are suitable for forming golf ball components and other golf-related equipment.